Re: [PATCH] refs: make sure we never pass NULL to hashcpy
- From
Michael Haggerty <mhagger@alum.mit.edu>
- Date
- Sep 7, 2017, 07:26 UTC
- Message-ID
- <CAMy9T_ED1KBqkE9GCHrOrt0frnYAx1vka7Xx1DrXmjJBNNKahw@mail.gmail.com>
- In-Reply-To
- <xmqqh8wgaa7s.fsf@gitster.mtv.corp.google.com>
On Wed, Sep 6, 2017 at 3:26 AM, Junio C Hamano <gitster@pobox.com> wrote:
Show 27 quoted lines
> Thomas Gummerer <t.gummerer@gmail.com> writes: > >> gcc on arch linux (version 7.1.1) warns that a NULL argument is passed >> as the second parameter of memcpy. >> [...] > > It is hugely annoying to see a halfway-intelligent compiler forces > you to add such pointless asserts. > > The only way the compiler could error on this is by inferring the > fact that new_sha1/old_sha1 could be NULL by looking at the callsite > in ref_transaction_update() where these are used as conditionals to > set HAVE_NEW/HAVE_OLD that are passed. Even if the compiler were > doing the whole-program analysis, the other two callsites of the > function pass the address of oid.hash[] in an oid structure so it > should know these won't be NULL. > > [...] > > I wonder if REF_HAVE_NEW/REF_HAVE_OLD are really needed in these > codepaths, though. Perhaps we can instead declare !!new_sha1 means > we have the new side and rewrite the above part to > > if (new_sha1) > hashcpy(update->new_oid.hash, new_sha1); > > without an extra and totally pointless assert()?
The ultimate reason for those flags is that `struct ref_update` embeds `new_oid` and `old_oid` directly in the struct, so there is no way to set it to "NULL". (The `is_null_sha1` value is used for a different purpose.) So those flags keep track of whether the corresponding value is specified or absent.
Four of the five callers of `ref_transaction_add_update()` are constructing a new `ref_update` from an old one. They currently don't have to look into `flags`; they just pass it on (possibly changing a bit or two). Implementing your proposal would oblige those callers to change from something like
Show 5 quoted lines
> new_update = ref_transaction_add_update( > transaction, "HEAD", > update->flags | REF_LOG_ONLY | REF_NODEREF, > update->new_oid.hash, update->old_oid.hash, > update->msg);
to
Show 6 quoted lines
> new_update = ref_transaction_add_update( > transaction, "HEAD", > update->flags | REF_LOG_ONLY | REF_NODEREF, > (update->flags & REF_HAVE_NEW) ? update->new_oid.hash : NULL, > (update->flags & REF_HAVE_OLD) ? update->old_oid.hash : NULL, > update->msg);
It's not the end of the world, but it's annoying. `ref_transaction_add_update()` was meant to be a low-level, low-overhead way of allocating a `struct ref_update` and add it to a transaction.
Another solution (also annoying, but maybe a tad less so) would be to change the one iffy caller, `ref_transaction_update()`, to pass in a pointer to the null OID for `new_sha1` and `old_sha1` when the corresponding flags are turned off. That value would never be looked at, but it would hopefully reassure gcc.
I did just realize one thing: `ref_transaction_update()` takes `flags` as an argument and alters it using
> flags |= (new_sha1 ? REF_HAVE_NEW : 0) | (old_sha1 ? REF_HAVE_OLD : 0);
Perhaps gcc is *more* intelligent than we give it credit for, and is actually worried that the `flags` argument passed in by the caller might *already* have one of these bits set. In that case `ref_transaction_add_update()` would indeed be called incorrectly. Does the warning go away if you change that line to
Show 8 quoted lines
> if (new_sha1) > flags |=REF_HAVE_NEW; > else > flags &= ~REF_HAVE_NEW; > if (old_sha1) > flags |=REF_HAVE_OLD; > else > flags &= ~REF_HAVE_OLD;
? This might be a nice change to have anyway, to isolate `ref_transaction_update()` from mistakes by its callers. For that matter, one might want to be even more selective about what bits are allowed in the `flags` argument to `ref_transaction_update()`'s callers:
> flags &= REF_ALLOWED_FLAGS; /* value would need to be determined */
Michael